The three methods of atomic power, thermal power, and hydraulic power are now used as main power generation methods, and from a viewpoint of resource quantity and energy density, the three power generation methods are also expected to be used as the main power generation methods in the future. Especially, since thermal power generation is safe, its utility value is high as a power generation method with high capacity to respond to load change, it is expected that thermal power generation also will also continue to play an important role in the power generation field in the future.
In coal-fired thermal power generation including steam turbines, improvement of efficiency has been advanced since before. Generally, power generation is now performed under steam conditions of 600° C. class or less, and high-chrome steels (ferritic heat-resisting steels), such as 12Cr steel which has thermal resistance to the steam temperature, are used for primary members, such as turbine rotors and moving blades.
Additionally, although power generation techniques which have adopted steam conditions of the 700° C. class have recently been demanded for CO2 emissions reduction and further improvements in thermal efficiency, when steam conditions of the 700° C. class are adopted, strength becomes insufficient in the high-chrome steels (ferritic heat resisting steels), such as the 12Cr steel.
Thus, it is conceivable that Ni-based alloys which have still higher high-temperature strength are applicable as material for turbine rotors. However, since the manufacturing of a large-sized ingot of Ni-based alloys is difficult, enlargement of the turbine rotor is difficult, and very expensive. Therefore, it is not realistic to manufacture the turbine rotor using only Ni-based alloys.
Thus, Patent Document 1 (Japanese Laid-Open Patent Application No. 2008-88525) discloses a turbine rotor provided in a steam turbine into which high-temperature steam of 650° C. or higher is introduced, as a turbine rotor which is made of Ni-based alloys but uses the Ni-based alloys for essential parts and uses iron and steel materials only for other parts. Here, the turbine rotor is constructed such that a part divided into a portion made of the Ni-based alloys and a portion made of CrMoV steels according to steam temperature is connected by welding, and the steam temperatures of a connecting portion between the portion made of the Ni-based alloy and the portion made of the CrMoV steels, and the steam temperature of the portion made of the CrMoV steels are maintained at 580° C. or lower. Additionally, the CrMoV steels include low CrMoV steels which contain 0.85 to 2.5% of Cr by weight %.
However, in the technique disclosed in Patent Document 1, the CrMoV steels include low CrMoV steels which contain 0.85 to 2.5% of Cr by weight %. Thus, in the low CrMoV steels, it is expected that thermal resistance is insufficient in parts other than the part made of the Ni-based alloys. Assuming that high-Chrome steels are used simply instead of said low CrMoV steels, difference in linear expansion coefficient between Ni-based alloys and high-Chrome steels is large and thermal stress in the welded joints will be increased. Therefore, maintaining the strength of the welded joints becomes difficult.
[Literature of prior art]: Patent literature 1 (Japanese Laid-Open Patent Application No. 2008-88525)